It is sought more particularly here below in this document to describe problems existing in the field of seismic data acquisition for oil prospecting industry. The disclosure of course is not limited to this particular field of application but is of interest for any technique that has to cope with closely related or similar issues and problems.
The operations of acquiring seismic data on site conventionally use networks of seismic sensors, like accelerometers, geophones or hydrophones. We consider below the context of seismic data acquisition in a marine environment, in which the seismic sensors are hydrophones. The hydrophones are distributed along cables in order to form linear acoustic antennas normally (also referred to as “streamers” or “seismic streamers”). As shown in FIG. 1, the network of seismic streamers 20a to 20e is towed by a seismic vessel 21. The hydrophones are referenced 16 in FIG. 2, which illustrates in detail the block referenced C in FIG. 1 (i.e. a portion of the streamer referenced 20a).
The seismic method is based on an analysis of reflected seismic waves. Thus, to collect geophysical data in a marine environment, one or more submerged seismic sources are activated in order to propagate omnidirectional seismic wave trains. The pressure wave generated by the seismic source passes through the column of water and insonifies the different layers of the seabed. The reflected seismic waves (i.e. reflected acoustic signals) are then detected by the hydrophones distributed over the length of the seismic streamers. These acoustic signals are processed and retransmitted by telemetry from the seismic streamers to the operator station situated on the seismic vessel, where the processing of the raw data is carried out (in an alternative solution, the seismic acoustic signals are stored for a later processing).
During seismic surveys, it is important to precisely locate the streamers in particular for:                monitoring the position of the hydrophones (distributed along the seismic streamers) in order to obtain a satisfactory precision of the image of the seabed in the exploration zone;        detecting the movements of the streamers with respect to one another (the streamers are often subjected to various external natural constrains of variable magnitude, such as the wind, waves, currents); and        monitoring the navigation of streamers, in particular in a situation of bypassing an obstacle (such as an oil barge).        
Control of the positions of streamers lies in the implementation of navigation control devices (also referred to as “birds”) (white squares referenced 10 in FIG. 1). They are installed at regular intervals (every 300 meters for example) along the seismic streamers. The function of those birds is to guide the streamers between themselves. In other words, the birds are used to control the depth as well as the lateral position of the streamers. For this purpose, and as illustrated in FIG. 2, each bird 10 comprises a body 11 equipped with motorized pivoting wings 12 (or more generally means of mechanical moving) making it possible to modify the position of the streamers laterally between them (this is referred to a horizontal driving) and drive the streamers in immersion (this is referred to a vertical driving).
Seismic marine acquisition is usually performed close to the coast where fishermen have installed fishing nets. Some areas over the world are full of thousands of nets, which are not identified. Despite fishermen are requested to remove the nets and chase boats try to detect presence of nets, they are still many nets at sea, when the seismic vessel is working. Seismic streamers towed by vessel can easily catch fishing nets. The slanted shape of the streamers, in the current acquisition programs, increases the risk for a streamer to catch nets that float below the surface because the water column is more important. Since streamers are smooth, nets (or any inconvenient foreign objects) are sliding down the streamer, until they get caught by a prominent device which most often is a bird.
When a fishing net is caught by a bird:                drag increase on the streamer;        noise increases and can exceed the maximum noise tolerance (i.e. noise is too high compared with the useful seismic signal), which generates downtime;        the bird can no more control depth with effect on the cable that can go out of the specified depth, which generates downtime;        the bird can be unstable and can twist, with effect on the streamer that can twist and be damaged, which generates downtime.        
There are several kinds of birds in the market, whose behavior is not the same in the presence of a net (or any inconvenient foreign object).
Birds of a first type (called “Bird In Line” or “BIL”) are placed in series with the streamer, using connectors that supply power and data communication. See for example the following patent documents: U.S. Pat. No. 7,933,163, U.S. Pat. No. 7,206,254, US2009/0204274 and U.S. Pat. No. 7,080,607. These “BIL” comprise a body, in series with the streamer, and wings, which are interdependent with the body. These birds comprise, for each wing, a passive mechanical fuse which breaks to prevent the transmission of excessive mechanical stress to the bird and hence to the streamer. If a fishing net is caught by a wing, only this wing is released.
Birds of a second type (called “Bird In Parallel” or “BIP”) are placed in parallel with the streamer, using quick latches attached to the streamer. See for example the following patent document: U.S. Pat. No. 6,525,992. Power is supplied by internal battery and data communication is done through contactless magnetic coils. If a fishing net is caught by a wing, quick latches are acting as passive mechanical fuse to release the entire bird (body and wings). A rope fixed to the streamer allows the bird to remain attached to the streamer when released.
Birds of a third type (e.g. Sercel's product called “Nautilus®” (registered trademark)) can be seen as a hybrid solution since they comprise (see bird 10 of FIG. 3): at least one module (not shown), placed in series along the streamer (not shown), and a body (tubular frame element) 11, freely rotating around the at least one module and placed in parallel with the streamer. Three wings are attached to the body 11: two motorized upper wings 12a, 12b and a lower non-motorized wing (keel) 12c. In a first embodiment, if a fishing net is caught by the bird, it is not expected to release one or several wings (there is a wing loss only by breaking). In a second embodiment, if a fishing net is caught by a wing, only this wing is released using a passive mechanical fuse (as for the birds of the first type).
All these birds (first, second or third type) have drawbacks.
Indeed, the use of a passive mechanical fuse (for a wing in the case of birds of the first or third type, or for the whole bird in the case of birds of the second type) is not a reliable solution since the passive mechanical fuse can be seized or neutralized depending how fishing nets (or any inconvenient foreign objects) are attached to the bird and/or to the streamer. This results in a non-activation of the passive mechanical fuse when required. Inversely, the passive mechanical fuse (for the entire bird or a given wing) can be activated when not required (e.g. in case of heavy seas, the entire bird or the given wing may be released due to a severe load on the wing caused by the swell, when the entire bird or the given wing should remain attached to the streamer).
Moreover, when the whole bird or a given wing is released, lateral and/or vertical controls (previously carried out by the bird) are lost or no more managed correctly.
The birds of the third type (“Nautilus®” solution) become unstable when a fishing net is caught in the wings. They roll over the stability domain and twist around the streamer. They finally twist also the streamer on which they are fixed, until damaging the streamer.